Pulling engines

[Warzouz]

If you set engine on top of a part, the real thrust is cancelled and you can even blow up parts.

Is there a length limit to this cancellation ? How far so I have to set the engine not to have the thrust cancelled and the bellow part blown ?

[Wemb]

I'm not sure that's going to be easy to answer - I suspect it will depend (definitely) on the heat tollerance of the part that's blowing up and on the heat output of the engine.  Best bet is probably experimentation. If you can, rotate your stages to avoid having engines firing directly on-top of items below them. If you want more specific advise, a screenshot of your rocket might help.

Wemb

[Snark]

3 hours ago, Warzouz said:

If you set engine on top of a part, the real thrust is cancelled and you can even blow up parts.

Well, yes, which makes physical sense if you think about it.  It's the same reason you can't lift yourself off the floor by pulling up on your pants. 

3 hours ago, Warzouz said:

Is there a length limit to this cancellation ? How far so I have to set the engine not to have the thrust cancelled and the bellow part blown ?

In the real world, no.  Rocket exhaust needs to be aimed somewhere that it won't hit anything, because rocket exhaust is, well, unpleasant.

In KSP, I believe that the answer is yes, there's a limit, though I think it depends on the engine-- how far that limit is will depend on the engine's model, IIRC.  (For example, I think I read a post from someone saying that the ion engine has a limit of zero, i.e. you can put it inside something and it'll work just fine, obstructing the exhaust causes no problems.)

In my own games, I generally build my ships so that the exhaust simply stays well clear of the rest of the ship-- both for the practical reason that you've run into, and also for the somewhat role-playing reason that it feels like "cheating" to me if I've built a physically impossible ship, even if the distance is such that the game doesn't care about my exhaust.  I want each engine's exhaust to have a clear, unobstructed line from the nozzle to infinity.

I usually accomplish this either by mounting the engines on outriggers so they're far enough out to one side that their exhaust doesn't hit the rest of the ship.  Occasionally I'll rotate them slightly to point the exhaust outwards-- that means taking some cosine loss, but it's very minimal as long as you keep the angle small.  For example, a 5-degree rotation will incur only a 0.4% cosine loss; 10 degrees will be only a 1.5% loss.  (After that it starts to build up in a hurry, so I try to avoid a deflection of more than 10 degrees.)  5 degrees may not sound like much, but it can give you a fair amount of elbow room if the ship is long.

[Red Iron Crown]

4 hours ago, Warzouz said:

Is there a length limit to this cancellation ? How far so I have to set the engine not to have the thrust cancelled and the bellow part blown ?

The length limit varies by engine, I believe it's determined by the thrust transform in the model.

Worth pointing out that some engines do not model thrust blocking at all (jet engines with thrust reversers, the ion engine, RCS thrusters).

[Gojira1000]

Ions don't model thrust blockage? Oh I need to abuse that.

[Warzouz]

50 minutes ago, Snark said:

Well, yes, which makes physical sense if you think about it.  It's the same reason you can't lift yourself off the floor by pulling up on your pants. 

In the real world, no.  Rocket exhaust needs to be aimed somewhere that it won't hit anything, because rocket exhaust is, well, unpleasant.

...

Well, I could simply add some cosinus loss and thrust outward. Of the pole is long reasonably long, loss should be minimal.

[Plusck]

When I was designing my asteroid miner, I read that this could be guestimated by the size of the exhaust cone.

It's easy enough to test, though. Radially attach the engine to a stack of girders, and place structural panels on the bottom underneath it. Right clicking the engine on the launch pad (or KER etc) will tell you its thrust.

From memory, I think I found that the LV-N's required distance was 3.5 long girders.